The different biosensing strategies with microring resonators consist in quantifying the resonance shift under thepresence of biological or chemical agents. These strategies lead to the measurement of a wavelength shift or an output power variation correlated to the analyte concentration. The design guidelines to enhance the sensitivity have been already studied and reviewed over the past decade. However, very few studies have been published addressing ring resonator performance used as absorption micro-spectrometer, which can be explored with colorimetric chemistry and microfluidic handling. In this work, a sensitivity model for the operation of absorptive based ring resonators is presented. Such a model is proposed to provide a guideline to optimize ring resonator design and parameters. Finally, 2.5 FDTD simulations of different ring geometries are presented and compared to the results obtained with the model.